1. Field of the Invention
The present invention relates to a method for producing a ceramic heater in which a conductive heating resistor is held on a substrate containing an insulating ceramic, a ceramic heater produced by the production method, and a glow plug comprising the ceramic heater.
2. Description of the Related Art
Conventionally, a glow plug used for assisting start of a diesel engine is configured by: a rod-like heater housing a heating element which generates heat when energized; a cylindrical member which surrounds and holds the heater such that a main heat generating portion of the heater protrudes from the tip end; a cylindrical metal shell which holds the outer circumference of the cylindrical member so that the tip end of the heater protrudes; and the like. In a ceramic heater, particularly, both electrodes or a cathode and an anode for applying a voltage to a heating resistor of a heating element are disposed on the rear end side. One of the electrodes is electrically connected to a metal shell, and the other electrode is electrically connected to a center pole which is insulated and held on the rear end side of the metal shell.
Recently, as this type of heater, a ceramic heater has been used in which a heating element is held on a substrate made of a ceramic that has excellent corrosion resistance. Such a ceramic heater is usually formed in the following manner. First, a planar body made of an insulating ceramic is formed by injection molding. Next, a heating element is sandwiched in the planar body, and the whole body is subjected to a pressing process so as to be integrally molded. A debinding process is conducted on the molded body, and the molded body is then fired. The outer circumference is polished so as to be shaped, whereby the ceramic heater is completed.
A conductive ceramic in a heating element has been proposed for use in such a ceramic heater. The ceramic-made heating element can also be formed by injection molding in the same manner as the substrate. For example, JP-A-2002-364842 discloses a configuration in which a heating resistor (resistance heater) that mainly generates heat, and two lead portions connected respectively to both electrodes of the heating resistor extending to the outside of a ceramic heater are integrally produced into a U-like shape.
3. Problems to be Solved by the Invention
Before the firing process, however, the ceramic-made heating element is in a state where a ceramic powder is compressed and packed (hereinafter, an element in such state is also referred to as an “element green body”), and hence is brittle. Particularly, a portion which is to become the heating resistor after the firing process supports the two lead portions after the firing process, and therefore a load due to the weight of the lead portions is applied to the portion which is to become the heating resistor. When the heating element is handled carelessly in the production process of a ceramic heater, consequently, there is a possibility of damage to the heating element.